I think that would cause issues during various security reviews, because it would imply that the browser is accepting expired certificates, even if there is a w

I think that would cause issues during various security reviews, because it would imply that the browser is accepting expired certificates, even if there is a warning. Also, CAs might not revoke expired certificates any more, as they are already expired, which hurts security as well if there is a reason to revoke the certificate, but no means to do so. With an extension, this feature can be introduced gently, without risking any security issues.